Sensor instrumentation

ABSTRACT

An instrument system includes a sensor that has a logarithmic response, an antilog circuit and output means connected to the output of the antilog circuit to provide, as a linear function signals representative of the constituent sensed by the sensor. Balance adjust means for inserting a first compensating function in an additive manner to the signal from the sensor is connected in circuit between the sensor and antilog circuit and slope adjust means for inserting a second compensating function to change the slope characteristic of the signal applied to the antilog circuit is connected in circuit between the balance adjust means and the antilog circuit.

United States Patent Blackmer SENSOR INSTRUMENTATION David E. Blackmer,Harvard, Mass.

[73] Assignee: Instrumentation Laboratory, Inc.,

Lexington, Mass.

[22] Filed: April 7, 1970 [21] Appl. No.: 27,198

[72] Inventor:

[52] US. Cl ..324/29, 204/195 P, 23/254 E [51] Int. Cl. ..G0ln 27/42[58] Field of Search....324/29, 30 R; 204/1 T, 195 B,

204/195 P; 23/230 B, 232 E, 254 E [451 Sept. 26, 1972 PrimaryExaminerMichael J. Lynch Att0rneyWillis M. Ertman [5 7] ABSTRACT Aninstrument system includes a sensor that has a logarithmic response, anantilog circuit and output means connected to the output of the antilogcircuit to provide, as a linear function signals representative of theconstituent sensed by the sensor. Balance adjust means for inserting afirst compensating function in an additive manner to the signal from thesensor is connected in circuit between the sensor and antilog circuitand slope adjust means for inserting a second compensating function tochange the slope characteristic of the signal applied to the antilogcircuit is connected in circuit between the balance adjust means and theantilog circuit.

12 Claims, 3 Drawing Figures SENSOR INSTRUMENTATION SUMMARY OF INVENTIONThis invention relates to electrical instrumentation, and moreparticularly to electrical instrumentation for processing signals havinglogarithmic characteristics.

As an example, an electrochemical sensor for indicating the partialpressure of carbon dioxide in a fluid sample has a logarithmic output.In instrumentation utilizing such a sensor, it is desirable, onoccasion, to provide a direct digital readout of the partial pressure ofcarbon dioxide in the fluid sample. A specific type of such a carbondioxide sensor is an electrochemical sensor that is an adaption of a pHelectrode and which employs a selectively permeable membrane acrosswhich the carbon dioxide constituent of the fluid sample permeates. Suchan electrochemical sensor is extremely sensitive, must be calibratedaccurately for accurate response and the readout should be linear,particularly useful in connection with digital readouts or displays.

It is an object of this invention to provide novel and improvedinstrumentation for providing a linear readout of a constituent sensedby a sensor that has a logarithmic response.

Another object of the invention is to provide a novel and improvedelectrochemical sensor system.

Still another object of the invention is to provide a novel and improvedelectrochemical sensor system that has reduced response time and whichis convenient to adjust and calibrate.

A further object of the invention is to provide novel and improvedelectrochemical sensor instrumentation for providing a linear indicationof the partial pressure of carbon dioxide in a blood sample.

In accordance with the invention there is provided an instrumentationsystem that employs a sensor having a logarithmic response, circuitryfor producing as an output signal as an antilogarithmic function of theinput signal applied to the circuit, output means connected to theoutput of the antilogarithmic circuit for providing, as a linearfunction, signals representative of the constituent sensed by thatsensor, balance adjust means connected in circuit between the sensor andthe antilogarithmic circuit, and slope adjustment means connected incircuit between the balance adjustment means and the antilogarithmiccircuit.

In a particular embodiment the sensor is an electrochemical sensor thatincludes an electrode assembly and a membrane permeable to carbondioxide gas. A differential amplifier circuit is connected to theelectrode assembly and an adjustable derivative circuit is connected tothe output of the differential amplifier. As the ion exchange processinvolved in this pCO measurement is slow, and the electrical inputs tothe measurement section are approximately exponential functions of time,the derivative circuit reduces the response time of the system byshaping the output signal from the differential amplifier. A firstoperational amplifier stage is also connected to the output of thedifferential amplifier and includes balance adjust circuitry whichinserts in series a voltage to compensate for small potentialdifferences that may be generated because of such things as curvature ofthe pH glass membrane of the electrode assembly. Slope adjust con-' trolis included in the feedback circuit of this operational amplifier stageto provide an adjustment of the gain of that amplifier stage. Thecircuitry is designed such that this first operational amplifiercircuit, with a predetermined signal from the sensor, has zero outputsignal at an isoelectric point at which the balance adjustment may bevaried independently of the slope adjust control. After the circuit hasbeen shifted to the isoelectric point, the gain of the circuitry may beadjusted by using the slope adjust control. The output of theoperational amplifier is applied to the antilog circuit which in thisparticular embodiment produces an output current that is anantilogarithmic function of an input voltage. A second operationalamplifier stage is connected with the output of the antilogarithmiccircuit for converting the output current to a voltage which is appliedto an analog to digital converter and from that converter to a digitaldisplay where the partial pressure of carbon dioxide in the sample beingsensed by the sensor is directly displayed in the digital form. Thiscircuitry enables efficient, rapid measurements of the partial pressureof carbon dioxide with accuracy and in anarrangement which may becalibrated easily through two sequential adjustments, the firstadjustment being independent of the second adjustment.

Other objects, features and advantages of the invention will be seen asthe following description of a particular embodiment progresses, inconjunction with the drawing, in which:

FIG. 1 is a schematic diagram of a sensor and readout system constructedin accordance with the invention; and

FIGS. 2 and 3 are graphs indicating the nature of the adjustment of thecircuitry shown in FIG. 1.

DESCRIPTION OF PARTICULAR EMBODIMENT With reference to FIG. 1, structure10 defines a sample chamber 12 having an inlet 14 and an outlet 16.Disposed for sensing the partialpressure of carbon dioxide in a fluidsample in chamber 12 is an electrode assembly 20 which includes a pHsensitive glass membrane 22 at its tip which forms an end wall of aninner chamber 24 in which is disposed a silver/silver chloride electrode26. An outer chamber 28 is filled with a pCO electrolyte and contains asilver/silver chloride reference electrode 30. Disposed over the end ofthe electrode assembly is a nylon mesh spacer membrane 32 secured byO-ring 34 and a selectively permeable silastic membrane 36 secured byO-ring 38. Membrane 36 is permeable to carbon dioxidegas but not toions.

Electrode 26 is connected to impedance transforming amplifier 50 andreference electrode 30 is connected to a similar impedance transformingamplifier 5 2. A network which includes resistor 54 establishesrelationship to ground for the electrode assembly 20'. Impedancetransforming amplifiers 50 and 52 are in turn connected to correspondingstages of a differential amplifier that includes operational amplifiers56 and 58, respectively. Potentiometer 60 provides a bi-lateraladjustment for common mode rejection and components 62, 64, 66 and 68are precision (0.02 percent) resistors.

A derivative compensation circuit that includes potentiometer 70,resistor 72 and capacitor 74' is connected to the output of thedifferential amplifier. Potentiometer provides an adjustment to vary thetime constant of the derivative compensation circuit.

Also connected to the output of the differential amplifier is anoperational amplifier stage that includes operational amplifier 76 withinput resistor 78 and a feedback circuit that includes resistor 80,smoothing capacitor 82 and slope adjustment potentiometer 84. Thepotentiometer 84 is connected to the junction of voltage divider networkthat includes resistors 86 and 88. A balance adjustment circuit includespotentiometer 90 connected to voltage sources as indicated and resistor92. Connected to the output of the operational amplifier stage is anantilog circuit 94 which converts an input voltage to an output current.In this particular embodiment antilog circuit 94 has the followingcharacteristics:

' The output of the antilog circuit 94 is connected to an operationalamplifier stage 96 with its feedback resistor 98 being of such valuethat, in response to a seventy microamperes input current, an outputvoltage is produced that generates at display 102 a reading of 35millimeters partial pressure. The output signal from amplifier 96 isapplied to analog to digital converter 100 which in turn is connected toa display 102 which displays in digital form the sensed partial pressureof carbon dioxide in the sample in chamber 12.

In this embodiment, the electrode assembly 20 is calibrated with twogases, a low gas mixture of about percent carbon dioxide, about 12percent oxygen and the balance nitrogen, and a high gas of about percentcarbon dioxide, and the remainder nitrogen. When the low gas supply isflowing through the sample chamber 12, electrode assembly has an outputof about +5 millivolts and differential amplifier provides an outputvoltage of about 75 millivolts. Balance potentiometer 90 is adjusted tointroduce a potential of appropriate polarity to balance that outputvoltage so that the input voltage to amplifier 76, and accordingly itsoutput voltage is zero. The antilogarithmic circuit 94 has a one decadeper volt response and operational amplifier stage 96 produces an outputsignal indicating a partial pressure of 35 millimeters Hg at display 102in response to an input current of 70 microamperes as indicated aboveand thus this adjustment shifts system response line 110 vertically asindicated in FIG. 2 until it intersects isoelectric point 112 (in thisembodiment 35 mm Hg). At that current output from antilog circuit 94,the input voltage is zero and in that condition the position of the tapof the slope potentiometer 84 has no effect. After the circuitry hasbeen balanced with the low gas, the high gas is flowed through samplechamber 12 and slope potentiometer 84 is adjusted to rotate the systemresponse line 110 about the isoelectricpoint 112 as indicated in FIG. 3until the display 102 indicates the partial pressure (a typical valuebeing 70 mm Hg) of the high gas. In this condition, system response line110 intersects the low gas point 112 and the high gas point 114 and thushas been calibrated.

After the system has been calibrated, a blood sample is introduced intosample chamber 12 and after about 40 seconds, (this delay being reducedabout forty seconds by the derivative circuit) the output is stabilizedand the digital reading in millimeters of mercury displayed by display102 indicates the partial pressure of carbon dioxide in the blood samplein chamber 12. Additional details of a blood analysis system in whichthis apparatus is incorporated are set out in copending application Ser.No. 27,200, now US. Pat. No. 3,658,478 entitled Fluid AnalyzingApparatus filed in the same of Spergel et al. and assigned to the sameassignee as this application.

While a particular embodiment of the invention has been shown anddescribed, various modifications thereof will be apparent to thoseskilled in the art and therefore it is not intended that the inventionbe limited to the disclosed embodiment or to details thereof anddepartures may be made therefrom within the spirit and scope of theinvention.

What is claimed is:

1. An instrumentation system for measuring the concentration of a gasconstituent in a fluid comprising sensor means responsive to said gasconstituent and having a logarithmic response, circuitry for producingas an output signal as an antilogarithmic function of the input signalapplied to the circuit, means for connecting said sensor to the input ofsaid antilogarithmic circuit, output means connected to the output ofsaid antilogarithmic circuit for providing, as a linear function,signals representative of the constituent sensed by said sensor, balanceadjust means connected in circuit between said sensor and saidantilogarithmic circuit for inserting a first compensating function inan additive manner to the signal from said sensor, and slope adjustmentmeans connected in circuit between said balance adjustment means andsaid antilogarithmic circuit for inserting a second compensatingfunction to change the slope characteristic of the signal applied tosaid antilogarithmic circuit.

2. An instrumentation system for measuring the concentration of a gasconstituent in a fluid comprising a sensor having a logarithmicresponse, said sensor being an electrochemical sensor that includes anelectrode assembly and a permeable membrane interposed between saidelectrode assembly and the fluid to be sensed, circuitry for producingas an output signal as an antilogarithmic function of the input signalapplied to the circuit, means for connecting said sensor to the input ofsaid antilogarithmic circuit, output means connected to the output ofsaid antilogarithmic circuit for providing, as a linear function,signals representative of the constituent sensed by said sensor, balanceadjust means connected in circuit between said sensor and saidantilogarithmic circuit for inserting a first compensating function inan additive manner to the signal from said sensor, and slope adjustmentmeans connected in circuit between said balance adjustment means andsaid antilogarithmic circuit for inserting a second compensatingfunction to change the slope characteristic of the signal applied tosaid antilogarithmic circuit.

3. The system as claimed in claim 2 wherein said means for connectingsaid sensor to the input of said antilog circuit includes a differentialamplifier circuit and further including an adjustable derivative circuitconnected to the output of the differential amplifier.

4. The system as claimed in claim 1 and further including a firstoperational amplifier stage connected between said sensor and saidantilogarithmic circuit and wherein said balance adjust means inserts inseries a voltage between said sensor and said first operationalamplifier stage.

5. The system as claimed in claim 4 wherein said slope adjustment meansis included in the feedback circuit of said first operational amplifierstage.

6. The system as claimed in claim 4 wherein said output means include asecond operational amplifier stage connected to the output of saidantilogarithmic circuit for converting the output current of saidantilogarithmic circuit to a voltage.

7. The system as claimed in claim 1 wherein said output means include ananalog to digital converter responsive to the output of saidantilogarithmic circuit and a digital display responsive to the outputof said converter.

8. An instrumentation system for measuring the concentration of a gasconstituent in a fluid comprising an electrochemical sensor responsiveto said gas constituent, a first operational amplifier stage connectedto the output of said sensor, balance adjust circuitry for inserting inseries a compensating voltage between said sensor and said firstoperational amplifier stage, slope adjust control connected in thefeedback circuit of said first operational amplifier stage to provideadjustment of the gain of that amplifier stage so that with apredetermined signal from said sensor said first operational amplifierstage has zero output signal at an isoelectric point at which saidbalance adjust circuitry may be varied independently of said slopeadjust control, and an antilog circuit connected to the output of saidfirst operational amplifier stage for providing signals representativeof the constituent sensed by said sensor.

9. The system as claimed in claim 8 and further including a secondoperational amplifier stage connected to the output of said antilogcircuit and an analog to digital converter connected to the output ofsaid second operational amplifier stage for applying an output signal toa digital display. v

10. The system as claimed in claim 8 wherein said slope adjust controlincludes a potentiometer and said circuitry is arranged so that zerovoltage is impressed across said potentiometer at said isoelectricpoint.

11. An instrumentation system for measuring the concentration of a gasconstituent in a fluid comprising an electrochemical sensor thatincludes an electrode assembly and a permeable membrane interposedbetween said electrode assembly and the fluid to be sensed, a firstoperational amplifier stage connected to the output of said sensor,balance adjust circuitry for inserting in series a compensating voltagebetween said sensor and said first operational amplifier stage, slopeadjust control connected in the feedback circuit of said firstoperational amplifier stage to provide adjustment of the gain of thatamplifier stage so that with a predetermined signal from said sensorsaid first operational amplifier state has zero output signal at anisoelectric point at which said balance adjust circuitry may be variedindependently of said slope adjust control, and an antilog circuitconnected to the output of said first operational amplifier stage forproviding signals representative of the constituent sensed by said sensr.

12? The system as claimed in claim 1 1 and further in-

1. An instrumentation system for measuring the concentration of a gasconstituent in a fluid comprising sensor means responsive to said gasconstituent and having a logarithmic response, circuitry for producingas an output signal as an antilogarithmic function of the input signalapplied to the circuit, means for connecting said sensor to the input ofsaid antilogarithmic circuit, output means connected to the output ofsaid antilogarithmic circuit for providing, as a linear function,signals representative of the constituent sensed by said sensor, balanceadjust means connected in circuit between said sensor and saidantilogarithmic circuit for inserting a first compensating function inan additive manner to the signal from said sensor, and slope adjustmentmeans connected in circuit between said balance adjustment means andsaid antilogarithmic circuit for inserting a second compensatingfunction to change the slope characteristic of the signal applied tosaid antilogarithmic circuit.
 2. An instrumentation system for measuringthe concentration of a gas constituent in a fluid comprising a sensorhaving a logarithmic response, said sensor being an electrochemicalsensor that includes an electrode assembly and a permeable membraneinterposed between said electrode assembly and the fluid to be sensed,circuitry for producing as an output signal as an antilogarithmicfunction of the input signal applied to the circuit, means forconnecting said sensor to the input of said antilogarithmic circuit,output means connected to the output of said antilogarithmic circuit forproviding, as a linear function, signals representative of theconstituent sensed by said sensor, balance adjust means connected incircuit between said sensor and said antilogarithmic circuit forinserting a first compensating function in an additive manner to thesignal from said sensor, and slope adjustment means connected in circuitbetween said balance adjustment means and said antilogarithmic circuitfor inserting a second compensating function to change the slopecharacteristic of the signal applied to said antilogarithmic circuit. 3.The system as claimed in claim 2 wherein said means for connecting saidsensor to the input of said antilog circuit includes a differentialamplifier circuit and further including an adjustable derivative circuitconnected to the output of the differential amplifier.
 4. The system asclaimed in claim 1 and further including a first operational amplifierstage connected between said sensor and said antilogarithmic circuit andwherein said balance adjust means inserts in series a voltage betweensaid sensor and said first operational amplifier stage.
 5. The system asclaimed in claim 4 wherein said slope adjustment means is included inthe feedback circuit of said first operational amplifier stage.
 6. Thesystem as claimed in claim 4 wherein said output means include a secondoperational amplifier stage connected to the output of saidantilogarithmic circuit for converting the output current of saidantilogarithmic circuit to a voltage.
 7. The system as claimed in claim1 wherein said output means include an analog to digital converterresponsive to the output of said antilogarithmic circuit and a digitaldisplay responsive to the output of said converter.
 8. Aninstrumentation system for measuring the concentration of a gasconstituent in a fluid comprising an electrochemical sensor responsiveto said gas constituent, a first operational amplifier stage connectedto the output of said sensor, balance adjust circuitry for inserting inseries a compensating voltage between said sensor and said firstoperational amplifier stage, slope adjust control connected in thefeedback circuit of said first operatiOnal amplifier stage to provideadjustment of the gain of that amplifier stage so that with apredetermined signal from said sensor said first operational amplifierstage has zero output signal at an isoelectric point at which saidbalance adjust circuitry may be varied independently of said slopeadjust control, and an antilog circuit connected to the output of saidfirst operational amplifier stage for providing signals representativeof the constituent sensed by said sensor.
 9. The system as claimed inclaim 8 and further including a second operational amplifier stageconnected to the output of said antilog circuit and an analog to digitalconverter connected to the output of said second operational amplifierstage for applying an output signal to a digital display.
 10. The systemas claimed in claim 8 wherein said slope adjust control includes apotentiometer and said circuitry is arranged so that zero voltage isimpressed across said potentiometer at said isoelectric point.
 11. Aninstrumentation system for measuring the concentration of a gasconstituent in a fluid comprising an electrochemical sensor thatincludes an electrode assembly and a permeable membrane interposedbetween said electrode assembly and the fluid to be sensed, a firstoperational amplifier stage connected to the output of said sensor,balance adjust circuitry for inserting in series a compensating voltagebetween said sensor and said first operational amplifier stage, slopeadjust control connected in the feedback circuit of said firstoperational amplifier stage to provide adjustment of the gain of thatamplifier stage so that with a predetermined signal from said sensorsaid first operational amplifier state has zero output signal at anisoelectric point at which said balance adjust circuitry may be variedindependently of said slope adjust control, and an antilog circuitconnected to the output of said first operational amplifier stage forproviding signals representative of the constituent sensed by saidsensor.
 12. The system as claimed in claim 11 and further including adifferential amplifier circuit connected between the output of saidelectrode assembly and said first operational amplifier stage, and anadjustable derivative circuit connected to the output of thedifferential amplifier for modifying the shape of the output signal fromsaid differential amplifier circuit, said modified output signal beingapplied to said first operational amplifier stage.